H2 Dissociation on H-Precovered Ni(100) Surface: Physisorbed State and Coverage Dependence

Hydrogen molecule dissociation on metal surfaces is a prototypical reaction for investigating the gas surface interaction. To investigate the effect of lattice motion, the embedded cluster model is adopted to construct the quantum Ni(100) lattice, in which 11 Ni atoms are treated quantum mechanically. The direct and steady-state dissociation rates of H-2 on H-precovered Ni(100) surface are calculated by quantum instanton method. Both the direct and steady-state dissociation rates on H-precovered Ni(100) are smaller than those on the clean Ni(100). This is because the repulsive interaction between H-2 and the preadsorbed H raises the potential energy barrier. Moreover, this repulsive interaction is inversely proportional to the distance between H-2 and the preadsorbed H. Owing to the classical relaxation and entropy effect of Ni atoms, the lattice motion promotes H-2 dissociation by lowering the free-energy barrier but it hinders H-2 recombination by raising the free-energy barrier. There are remarkable kinetic isotope effects for the dissociation process, which is due to the entropy and quantum tunneling effects. However, no kinetic isotope effect is obtained for the recombination process.